Thursday Jun 18, 2009

TLS and NIST'S Policy on Hash Functions

NIST'S Policy on Hash Functions

March 15, 2006: The SHA-2 family of hash functions (i.e., SHA-224, SHA-256, SHA-384 and SHA-512) may be used by Federal agencies for all applications using secure hash algorithms. Federal agencies should stop using SHA-1 for digital signatures, digital time stamping and other applications that require collision resistance as soon as practical, and must use the SHA-2 family of hash functions for these applications after 2010. After 2010, Federal agencies may use SHA-1 only for the following applications: hash-based message authentication codes (HMACs); key derivation functions (KDFs); and random number generators (RNGs). Regardless of use, NIST encourages application and protocol designers to use the SHA-2 family of hash functions for all new applications and protocols.

TLS specifics of hash functions

  1. MAC constructions

    A number of operations in the TLS record and handshake layer require a keyed Message Authentication Code (MAC) to protect message integrity or to construct key derivation functions.

    For TLS 1.0 and 1.1, the construction used is known as HMAC; TLS 1.2 still use HMAC, but it also decalares that "Other cipher suites MAY define their own MAC constructions, if needed."

  2. HMAC at handshaking

    HMAC can be used with a variety of different hash algorithms. However, TLS 1.0 and TLS 1.1 use it in the handshaking with two different algorithms, MD5(HMAC_MD5) and SHA-1(HMAC-SHA). Additionla hash algorithm can be defined by cipher suites and used to protect record data, but MD5 and SHA-1 are hard coded into the description of the handshaking for TLS 1.0 and TLS 1.1.

    TLS 1.2 move away from the hard coded MD5 and SHA-1, SHA-256 is the default hash function for all cipher suites defined in TLS 1.2, TLS 1.1, TLS 1.0 when TLS 1.2 is negotiated. TLS 1.2 also declares that "New cipher suites MUST explicitly specify a PRF and, in general, SHOULD use the TLS PRF with SHA-256 or a stronger standard hash function", which means that the hash functions used at handshakeing should be SHA-256 at least.

  3. HMAC at protecting record

    For the HMAC operations used to protect record data, the hash funtion is defined by cipher suites. For example, the HMAC's hash function of cipher suite TLS_RSA_WITH_NULL_MD5 is MD5.

    TLS 1.0 and TLS 1.1 define three hash functions for HMAC, they are:

    • null
    • MD5
    • SHA1

    From TLS 1.2, new cipher suites may define their own MAC constructions except the default HMAC. TLS 1.2 defines five MAC algorithms, from the literal, it is straight forward to know the hash function used.

    • null
    • hmac_md5
    • hmac_sha1
    • hmac_sha256
    • hmac_sha384
    • hmac_sha512

  4. Pseudo-Random Function

    Pseudo-random function takes a key rule in TLS handshaking, it is used to calculate the master secret, calculate session keys, and verify the just negotiated algorithms via Finished message. TLS specifications define PRF based on HMAC.

    For TLS 1.0 and TLS 1.1, the PRF is created by splitting the secret into two halves and using one half to generate data with P_MD5 and the other half to generate data with P_SHA-1, then exclusive-ORing the outputs of these two expansion functions together.

    PRF(secret, label, seed) = P_MD5(S1, label + seed) XOR P_SHA-1(S2, label + seed);

    TLS 1.2 defines a PRF based on HMAC as TLS 1.0/1.1, except that the hash algorithm used if SHA-256, "This PRF with the SHA-256 hash function is used for all cipher suites defined in this document and in TLS documents published prior to this document when TLS 1.2 is negotiated. New cipher suites MUST explicitly specify a PRF and, in general, SHOULD use the TLS PRF with SHA-256 or a stronger standard hash function."

    Unlike TLS 1.0/1.1, the PRF of TLS 1.2 does not require to split the secret any more, only one hash function used:

    PRF(secret, label, seed) = P_<hash>(secret, label + seed)

  5. Hash function at ServerKeyExchange

    In the handshakeing message, ServerKeyExchage, for some exchande method, such as RSA, diffie_hellman, ec_diffie_hellman, ecdsa, etc., needs a so-called "signature" to protect the exchanged parameters.

    TLS 1.0 and TLS 1.1 use SHA-1 ( or with MD5 at the same time) to generate the digest for the "signature". While for TLS 1.2, the hash function may be other than SHA-1, it is varied with the ServerKeyExchange message context, such as "signature algorithm" extension, the server end-entity certificate.

  6. Server Certificates

    In TLS 1.0/1.1, there is no way for client to indicate the server what kind of server certificates it would accept. TLS 1.2 defines a extension, signature_algorithms, to indicate to the server which signature/hash algorithm pairs may be used in digital signatures. The hash algorithm could be one of:

    • none
    • md5
    • sha1
    • sha224
    • sha256
    • sha384
    • sha512

  7. Client Certificates

    In TLS 1.0/1.1, a TLS server could request a serial of types of client certificate, but the "type" here refer to the "signature" algorithm, which does not include the hash algorithm the certificate should be signed with. So a certificate signed with a stonger signature algorithm, such as RSA2048, but with a weak hash funtion, such as MD5, would meet the requirements. That's not enough.

    TLS 1.2 extends the CertificateRequest handshaking message with a addtional field, "supported_signature_algorithms", to indicate to the client which signature/hash algorithm pairs may be used in digital signatures. The hash algorithm could be one of:

    • none
    • md5
    • sha1
    • sha224
    • sha256
    • sha384
    • sha512

What the FIPS 140-2 Concern

In the last update of "Implementation Guidance for FIPSPUB 140-2", "The KDF in TLS is allowed only for the purpose of establishing keying material (in particular, the master secret) for a TLS session with the following restrictions, even though the use of the SHA-1 and MD5 hash functions are not consistent with in Table 1 or Table 2 of SP 800-56A: "

  1. The use of MD5 is allowed in the TLS protocol only; MD5 shall not be used as a general hash function.
  2. The maximum number of blocks of secret keying material that can be produced by repeated use of the pseudorandom function during a single call to the TLS key derivation function shall be 2\^32-1.

NIST's Policy Compliant profile for TLS

The NIST's policy on hash functions could be split into four principle. We discuss the profile according to the principles.

  • Principle 1: The SHA-2 family of hash functions (i.e., SHA-224, SHA-256, SHA-384 and SHA-512) may be used by Federal agencies for all applications using secure hash algorithms.

    MD5 is not a FIPS approved hash functions, so first of all, the profile needs to disable all cipher suites with the MACAlgorith of MD5.

    • TLS_RSA_WITH_NULL_MD5
    • TLS_RSA_EXPORT_WITH_RC4_40_MD5
    • TLS_RSA_WITH_RC4_128_MD5
    • TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5
    • TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
    • TLS_DH_anon_WITH_RC4_128_MD5
    • TLS_KRB5_WITH_DES_CBC_MD5
    • TLS_KRB5_WITH_3DES_EDE_CBC_MD5
    • TLS_KRB5_WITH_RC4_128_MD5
    • TLS_KRB5_WITH_IDEA_CBC_MD5
    • TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5
    • TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5
    • TLS_KRB5_EXPORT_WITH_RC4_40_MD5

    SHA-2 family of hash functions are completely compliant to the policy. The profile is safe to enabled the those cipher suites based on SHA-2

    • TLS_RSA_WITH_NULL_SHA256
    • TLS_RSA_WITH_AES_128_CBC_SHA256
    • TLS_RSA_WITH_AES_256_CBC_SHA256
    • TLS_DH_DSS_WITH_AES_128_CBC_SHA256
    • TLS_DH_RSA_WITH_AES_128_CBC_SHA256
    • TLS_DHE_DSS_WITH_AES_128_CBC_SHA256
    • TLS_DHE_RSA_WITH_AES_128_CBC_SHA256
    • TLS_DH_DSS_WITH_AES_256_CBC_SHA256
    • TLS_DH_RSA_WITH_AES_256_CBC_SHA256
    • TLS_DHE_DSS_WITH_AES_256_CBC_SHA256
    • TLS_DHE_RSA_WITH_AES_256_CBC_SHA256
    • TLS_DH_anon_WITH_AES_128_CBC_SHA256
    • TLS_DH_anon_WITH_AES_256_CBC_SHA256
    • TLS_RSA_WITH_AES_128_GCM_SHA256
    • TLS_RSA_WITH_AES_256_GCM_SHA384
    • TLS_DHE_RSA_WITH_AES_128_GCM_SHA256
    • TLS_DHE_RSA_WITH_AES_256_GCM_SHA384
    • TLS_DH_RSA_WITH_AES_128_GCM_SHA256
    • TLS_DH_RSA_WITH_AES_256_GCM_SHA384
    • TLS_DHE_DSS_WITH_AES_128_GCM_SHA256
    • TLS_DHE_DSS_WITH_AES_256_GCM_SHA384
    • TLS_DH_DSS_WITH_AES_128_GCM_SHA256
    • TLS_DH_DSS_WITH_AES_256_GCM_SHA384
    • TLS_DH_anon_WITH_AES_128_GCM_SHA256
    • TLS_DH_anon_WITH_AES_256_GCM_SHA384
    • TLS_PSK_WITH_AES_128_GCM_SHA256
    • TLS_PSK_WITH_AES_256_GCM_SHA384
    • TLS_DHE_PSK_WITH_AES_128_GCM_SHA256
    • TLS_DHE_PSK_WITH_AES_256_GCM_SHA384
    • TLS_RSA_PSK_WITH_AES_128_GCM_SHA256
    • TLS_RSA_PSK_WITH_AES_256_GCM_SHA384
    • TLS_PSK_WITH_AES_128_CBC_SHA256
    • TLS_PSK_WITH_AES_256_CBC_SHA384
    • TLS_PSK_WITH_NULL_SHA256
    • TLS_PSK_WITH_NULL_SHA384
    • TLS_DHE_PSK_WITH_AES_128_CBC_SHA256
    • TLS_DHE_PSK_WITH_AES_256_CBC_SHA384
    • TLS_DHE_PSK_WITH_NULL_SHA256
    • TLS_DHE_PSK_WITH_NULL_SHA384
    • TLS_RSA_PSK_WITH_AES_128_CBC_SHA256
    • TLS_RSA_PSK_WITH_AES_256_CBC_SHA384
    • TLS_RSA_PSK_WITH_NULL_SHA256
    • TLS_RSA_PSK_WITH_NULL_SHA384
    • TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256
    • TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384
    • TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256
    • TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384
    • TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
    • TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384
    • TLS_ECDH_RSA_WITH_AES_128_CBC_SHA256
    • TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384
    • TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
    • TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
    • TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256
    • TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384
    • TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
    • TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
    • TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256
    • TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384
    • TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256
    • TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384
    • TLS_ECDHE_PSK_WITH_NULL_SHA256
    • TLS_ECDHE_PSK_WITH_NULL_SHA384

    Those cipher suites with MAC algorithm of SHA-1 are addressed at the follow principles.

  • Principle 2: Federal agencies should stop using SHA-1 for digital signatures, digital time stamping and other applications that require collision resistance as soon as practical, and must use the SHA-2 family of hash functions for these applications after 2010.
    Profile ServerKeyExchange Message

    ServerKeyExchange depends on digital signature, the profile should stop using SHA-1 hash function for ServerKeyExchange handshaking message.

    TLS 1.0 and TLS 1.1 use SHA-1 ( or with MD5 at the same time) to generate the digest for the "signature". There is no way to disable SHA-1 in ServerKeyExchange handshaking message. ServerKeyExchange is a optional handshaking message," it is sent by the server only when the server certificate message (if sent) does not contain enough data to allow the client to exchange a premaster secret. This is true for the following key exchange methods:"

    • RSA_EXPORT (if the public key in the server certificate is longer than 512 bits)
    • DHE_DSS
    • DHE_DSS_EXPORT
    • DHE_RSA
    • DHE_RSA_EXPORT
    • DH_anon

    For TLS 1.0 and TLS 1.1, the profile needs to disable the above key exchange methods, for the purpose of preventing the ServerKeyExchange handshaking message occurred, by disabling the following cipher suites:

    • TLS_RSA_EXPORT_WITH_RC4_40_MD5
    • TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5
    • TLS_RSA_EXPORT_WITH_DES40_CBC_SHA
    • TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA
    • TLS_DHE_DSS_WITH_DES_CBC_SHA
    • TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA
    • TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
    • TLS_DHE_RSA_WITH_DES_CBC_SHA
    • TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA
    • TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
    • TLS_DH_anon_WITH_RC4_128_MD5
    • TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA
    • TLS_DH_anon_WITH_DES_CBC_SHA
    • TLS_DH_anon_WITH_3DES_EDE_CBC_SHA
    • TLS_DHE_DSS_WITH_AES_128_CBC_SHA
    • TLS_DHE_RSA_WITH_AES_128_CBC_SHA
    • TLS_DH_anon_WITH_AES_128_CBC_SHA
    • TLS_DHE_DSS_WITH_AES_256_CBC_SHA
    • TLS_DHE_RSA_WITH_AES_256_CBC_SHA
    • TLS_DH_anon_WITH_AES_256_CBC_SHA

    In TLS 1.2, the hash function used with ServerKeyExchange may be other than SHA-1, the following rules defined:

    • Signature Algorithm Extension: If the client has offered the "signature_algorithms" extension, the signature algorithm and hash algorithm used in ServerKeyExchange message MUST be a pair listed in that extension.

      Per this rule, the profile requires that the "signature_algorithms" extension sent by client should include only SHA-2 hash algorithms or stronger, and must not include the hash algorithms: "none", "md5", and "sha1".

    • Compatible with the Key in Server's EE Certificate: the hash and signature algorithms used in ServerKeyExchange message MUST be compatible with the key in the server's end-entity certificate.

      Per this rule, the profile requires that the server end-entity certificate must be signed with SHA-2 or stronger hash functions.

      Note that, at present, the DSA(DSS) may only be used with SHA-1, the profile will not allow server end-entity certificate signed with DSA(DSS).

    Profile Server Certificate

    In TLS 1.0/1.1, there is no way for client to indicate the server what kind of server certificates it would accept. What we can do here is from the point of programming and managerment, the profile requires all server certificates must be signed with SHA-2 or stronger hash functions, and carefully checking that there is no certificate in the chain signed with none SHA-2-or-stronger hash functions.

    In TLS 1.2, there is a protocol specified behavior, "signature_algorithms" extension. "If the client provided a 'signature_algorithms' extension, then all certificates provided by the server MUST be signed by a hash/signature algorithm pair that appears in that extension." Per the specific, the profile requires that the "signature_algorithms" extension sent by client should include only SHA-2 hash algorithms or stronger, and must not include the hash algorithms: "none", "md5", and "sha1"

    However, "signature_algorithms" extension is not a mandatory extension in TLS 1.2, while server does not receive the "signature_algorithms" extension, it also needs to ship the NIST principle. So the profile still requires all server certificates must be signed with SHA-2 or stronger hash functions from the point of programming and management.

    Profile Client Certificate

    In TLS 1.0/1.1, there is no way for server to indicate the client it would accept what kind of hash algorithm used to signed the client certificates. What we can do here is from the point of programming and managerment, the profile requires all client certificates must be signed with SHA-2 or stronger hash functions.

    TLS 1.2 extends the CertificateRequest handshaking message with a addtional field, "supported_signature_algorithms", to indicate to the client which signature/hash algorithm pairs may be used in digital signatures. The profile requires that the "supported_signature_algorithms" field must include only SHA-2 hash algorithms or stronger, and must not include the hash algorithms: "none", "md5", and "sha1".

  • Principle 3: After 2010, Federal agencies may use SHA-1 only for the following applications:
    • hash-based message authentication codes (HMACs);
    • key derivation functions (KDFs);
    • random number generators (RNGs).

    Except the ServerKeyExchange, server Certificate and client Certificate messages, the hash function used in TLS protocols is for HMAC, KDF or RNG, which is allowed by the policy. Need no addtional profile for this principle.

  • Principle 4: Regardless of use, NIST encourages application and protocol designers to use the SHA-2 family of hash functions for all new applications and protocols.
  • TLS 1.0 and TLS 1.1 is totally depends on SHA-1 and MD5, there is no way to obey this principle. In order to fully remove the dependency on SHA-1/MD5, one have to upgrade to TLS 1.2 or later revisions.

A stric mode profile

  1. Disable all cipher suites which mac algorithm is MD5;
  2. Disable all cipher suites which may trigger ServerKeyExchange message;
  3. Accept only those certificates that signed with SHA-1 or stronger hash functions;
  4. Upgrade to TLS 1.2 for purpose of fully remove the dependence on weak hash functions.

Put it into practice

Currently, Java SDK does not support TLS 1.1 or later. The proposals talked here are for TLS 1.0, which is implemented by the default SunJSSE provider.

  1. Disable cipher suite

    JSSE has no APIs to disable a particular cipher suite, but there are APIs to set which cipher suites could be used at handshaking. Refer to SSLSocket.setEnabledCipherSuites(String[] suites), SSLServerSocket.setEnabledCipherSuites(String[] suites), SSLEngine.setEnabledCipherSuites(String[] suites) for detailed usage.

    By default, SunJSSE enables both MD5 and SHA-1 based cipher suites, and those cipher suites that trigger ServerKeyExchange massage. In FIPS mode, SunJSSE enable SHA-1 based cipher suites only, however some of those cipher suites that trigger ServerKeyExchange also enabled. So, considering the above strict mode profile, the coder must explicit call SSLX.setEnabledCipherSuites(String[] suites), and the parameter "suites" must not include MD5 based cipher suites, and those cipher suites triggering handshaking message, ServerKeyExchange.

  2. Constrain certificate signature algorithm
  3. The strict profile suggest all certificates should be signed with SHA-2 or stronger hash functions. In JSSE, the processes to choose a certificate for the remote peer and validate the certificate received from remote peer are controlled by KeyManager/X509KeyManager and TrustManager/X509TrustManager. By default, the SunJSSE provider does not set any limit on the certificate's hash functions. Considerint the above strict profile, the coder should customize the KeyManager and TrustManager, and limit that only those certificate signed with SHA-2 or stronger hash functions are available or trusted.

    Please refer to the section of X509TrustManager Interface in JSSE Reference Guide for details about how to customize trust manager by create your own X509TrustManager; and refer to the section of X509KeyManager Interface in JSSE Reference Guide for details about how to customize key manager by create your own X509KeyManager

Note that the above profile and suggestions are my personal understanding of the NIST's policy and TLS, they are my very peronal suggestions, instead of official proposals from Sun.

Linkage to the blog entry at simabc.blogspot.com

Tuesday Jun 16, 2009

JSSE Troubleshooting: Certificates Order in TLS Handshaking

Issue:

Failed with a exception: java.security.cert.CertPathValidatorException: subject/issuer name chaining check failed.

Example:

Test case:
     1  //
     2  // JSSE Troubleshooting: Disordered Certificate List in TLS Handshaking
     3  //
     4  import java.net.\*;
     5
     6  public class DisorderedCertificateList {
     7      public static void main(String[] Arguments) throws Exception {
     8          URL url = new URL("https://myservice.example.com/");
     9          URLConnection connection = url.openConnection();
    10
    11          connection.getInputStream().close();
    12      }
    13  }
	
Test environment:

The HTTPS server, myservice.example.com, is configurated with a certificate path that the certificates in the path is out of order. For example, the expected certificate path is server_certificate -> intermediate ca -> seld-signed root ca. However, the certificate path is configurated as server_certificate -> seld-signed root ca -> intermediate ca.

Test Result:
	Exception in thread "main" javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path validation failed: java.security.cert.CertPathValidatorException: subject/issuer name chaining check failed
		at sun.security.ssl.Alerts.getSSLException(Alerts.java:192)
		at sun.security.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1627)
		at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:204)
		at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:198)
		at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:994)
		at sun.security.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:142)
		at sun.security.ssl.Handshaker.processLoop(Handshaker.java:533)
		at sun.security.ssl.Handshaker.process_record(Handshaker.java:471)
		at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:904)
		at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1132)
		at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1159)
		at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1143)
		at sun.net.www.protocol.https.HttpsClient.afterConnect(HttpsClient.java:423)
		at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:185)
		at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:997)
		at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:254)
		at DisorderedCertificateList.main(DisorderedCertificateList.java:11)
	Caused by: sun.security.validator.ValidatorException: PKIX path validation failed: java.security.cert.CertPathValidatorException: subject/issuer name chaining check failed
		at sun.security.validator.PKIXValidator.doValidate(PKIXValidator.java:266)
		at sun.security.validator.PKIXValidator.doValidate(PKIXValidator.java:249)
		at sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:172)
		at sun.security.validator.Validator.validate(Validator.java:235)
		at sun.security.ssl.X509TrustManagerImpl.validate(X509TrustManagerImpl.java:147)
		at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:230)
		at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:270)
		at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:973)
		... 12 more
	Caused by: java.security.cert.CertPathValidatorException: subject/issuer name chaining check failed
		at sun.security.provider.certpath.PKIXMasterCertPathValidator.validate(PKIXMasterCertPathValidator.java:153)
		at sun.security.provider.certpath.PKIXCertPathValidator.doValidate(PKIXCertPathValidator.java:321)
		at sun.security.provider.certpath.PKIXCertPathValidator.engineValidate(PKIXCertPathValidator.java:186)
		at java.security.cert.CertPathValidator.validate(CertPathValidator.java:267)
		at sun.security.validator.PKIXValidator.doValidate(PKIXValidator.java:261)
		... 19 more
	

Cause:

Per the TLS specification (page 39, section 7.4.2, RFC2246), the certificate list passed to server Certificate message or client Certificate message "is a sequence (chain) of X.509v3 certificates. The sender's certificate must come first in the list. Each following certificate must directly certify the one preceding it."

So, the certificate order of the above test case, server_certificate -> seld-signed root ca -> intermediate ca, is not a TLS specification compliant behavior, the TLS handshaking is expected to fail.

Solution:

Checking the TLS/SSL configuration, and make sure that the certificate list sent to peer is properly configuated and in order.


Linkage to the blog entry at simabc.blogspot.com

Friday Jun 12, 2009

RSA AlgorithmIdentifier of X.509 Certificate

By far, RSA is a most wide used cryptography algorithm. Both ITU-T X.509 and IETF PKIX WG define the RSA algorithm identifier, however, they are not identical.

ITU-T X.509[1] defines the algorithm as:

rsa ALGORITHM ::= {
    KeySize
    IDENTIFIED BY  id-ea-rsa
}

KeySize ::= INTEGER

id-ea-rsa OBJECT IDENTIFIER ::= {joint-iso-itu-t(2) ds(5)
algorithm(8) encryptionAlgorithm(1) rsa(1)}

While IETF PKIX WG[2] defines the algorithm as:
rsaPublicKey ALGORITHM-ID ::= { OID rsaEncryption PARMS NULL }

rsaEncryption OBJECT IDENTIFIER ::= {iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) rsaEncryption(1)}
 
  

There two differences:
1. different OID.
    ITU-T defines it as "2.5.8.1.1", while PKIX WG defines it as "1.2.840.113549.1.1.1"

2. different algorithm parameters
    ITU-T defines a parameter for RSA, "KeySize", while PKIX WG defines it as null.

Indeed, the RSA encryption algorithm PKIX WG used is defined by PKCS#1 [3][4], it is the industry standard definition. Most of the world use PKCS#1 OID, but not the one of ITU-T. Because of the above differences, there is a risk of interoperability problems between ITU-T X.509 compliant implementations and PKIX compliant implementations.

Before JDK 7, Sun certificate implementation cannot recognize the ITU-T X.509 OID, "2.5.8.1.1", throws a java.security.InvalidKeyException instead. It would be get fixed at OpenJDK 7 M4. If you happened to have such similar interoperability problem, I'd appreciate it if you comment it here or mail me your problems.

Linkage to the blog entry at simsbc.blogspot.com

[1] http://www.itu.int/ITU-T/asn1/database/itu-t/x/x509/2008/AlgorithmObjectIdentifiers.html#AlgorithmObjectIdentifiers.rsa
[2] http://www.ietf.org/rfc/rfc2459.txt
[3] http://www.rsa.com/rsalabs/node.asp?id=2125
[4] http://www.ietf.org/rfc/rfc2459.txt

Thursday May 28, 2009

Understanding Self-Issued Certificate

Certificate Types

RFC5280 categorize certificate into two classes: CA certificates and end entity certificates, and CA certificates are divided into three classes: cross-certificates, self-issued certificates, and self-signed certificates.

certificate +- CA certificate
+- cross-certificate
+- self-issued certificate
+- self-signed certificat
     +- end entity certificate

"Cross-certificates are CA certificates in which the issuer and subject are different entities. Cross-certificates describe a trust relationship between the two CAs." [RFC5280]

"Self-issued certificates are CA certificates in which the issuer and subject are the same entity. Self-issued certificates are generated to support changes in policy or operations." [RFC5280]

"Self-signed certificates are self-issued certificates where the digital signature may be verified by the public key bound into the certificate. Self-signed certificates are used to convey a public key for use to begin certification paths." [RFC5280]

Self-signed certificates are speicial slef-issied certificates, so we also can redraw the above tree as:

certificate +- CA certificate
+- cross-certificate
+- self-issued certificate
+- self-signed certificat
     +- end entity certificate

Notes of Self-Signed Certificate

1. "The trust anchor information may be provided to the path processing procedure in the form of a self-signed certificate." [RFC5280]

2. "When the trust anchor is provided in the form of a self-signed certificate, this self-signed certificate is not included as part of the prospective certification path." [RFC5280]

Note of Self-Issued Certificate

1. "Name constraints are not applied to self-issued certificates (unless the certificate is the final certificate in the path)." [RFC5280]

2. "However, a CA may issue a certificate to itself to support key rollover or changes in certificate policies. These self-issued certificates are not counted when evaluating path length or name constraints."

3. The pathLenConstraint field of basic constrains extension "gives the maximum number of non-self-issued intermediate certificates that may follow this certificate in a valid certification path."

4. The valude of inhibit anyPolicy extension "indicates the number of additional non-self-issued certificates that may appear in the path before anyPolicy is no longer permitted."

Identify a Self-Issued Certificate

RFC5280 requires that if the names in the issuer and subject field in a certificate match according to the comparison rules of internationalized names in distingushed names, then the certificate is self-issued. Please refer to section 7.1 of [RFC5280] about the comparison rules.

However, RFC3280 does not define the comparison rules, which requires that, "A certificate is self-issued if the DNs that appear in the subject and issuer fields are identical and are not empty." The specificate implies a binary comparison of the subject and issuer fields.

I think a good practice would have the same binary subject and issuer fields while issue a self-issued certificate.

Identify a Self-Signed Certificate

Sounds like a stupid title, just as its name implies, it is self signed, so it is self identifiable, i.e., the public key bound into the certificate could be used to verify the digital signature of the same certificate. Definitely, it's true. OK, we get a bi-steps process:

1. identify that a certificate is a self-issued certificate;
2. Verify the certificate digital signature with the public key bound.

That's a precious process, but not a effective process. Digital signature verify normally hurts a lot of performance, and generally it is not needed to verify the digital signature during build a prospective certification path.

Identify a Self-Signed Certificate Effectively

Self-signed, in another words, the key bound into the certificate is the same as the key used to sign the certificate. Could we identify it by comparing the key bound and the key used to generate the certificate signature? Here comes the authority key identifier extension and the subject key identifier extension, refer to RFC3280/RFC5280 for details.

To facilitate certification path construction, the specification requires that the authority key identifier extension and the subject key identifier extension MUST be appear in all conforming CA certificate. "There is one exception; where a CA distributes its public key in the form of a "self-signed" certificate, the authority key identifier MAY be omitted."

With the help of the two key identifier extensions, we get the following steps:

1. identify that a certificate is a self-issued certificate;
2. for conforming CAs, if the subject key identifier extension appears, but no authority key identifier extension, it is a self-signed certificate; if the both appear, it is a self-signed certificate when the KeyIdentifier is identical.
3. for non-conforming CAs, Verify the certificate digital signature with the public key bound.

Suggested Practices:

1. Always issue certificate with subject key identifier extension and authority key identifier extension.

2. Always include the keyIdentifier field in the authority key identifier extension.

3. Always have the same binary subject and issuer fields while issue a self-issued certificate.

4. Only issue self-issued certificate as CA certificate.

5. For TLS, always send the intermediate self-issued certificate within the response certificate list, otherwise, the recepient normally cannot build a certification path to its trust anchors.

The Problemtic Practices Encountered

1. A self-signed CA certificate issues 1+ self-issued end entity certificates.

There is no problem to issue such self-issued End-Entity certificate, but I'm afraid many PKIX libraries would not be able to handle it properly. If your application dependents on third party's PKIX library, and if you have to issue such certificates, please do check the library and make sure it supports such cases.

2. A self-signed CA certificate issues a self-issued certificate as an indirect CRL issuer.

It is special example of self-issed end-entity certificate. Some CRL verification library cannot handle such a indirect CRL issuer correctly, please double check the library to make sure such indirect CRL issuer is supported.

Java SE SDK support the above two problemtic cases at OpenJDK 7 build 60. If you application have to support above cases, you need OpenJDK 7 build 60 at least.

Tuesday Dec 30, 2008

Understanding TLS protocol -- Certificate Status Request

For better understanding TLS protocol extensions, I draw a few sequence diagrams of TLS handshaking with extension, and marked the differences from the normal handshaking processes. Share them now. For legible image, please open the following image in new page or download the raw image from here.

TLS Handshaking, Certificate Status Request 

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