mbedtls/docs/architecture/psa-migration/psa-limitations.md

This document lists current limitations of the PSA Crypto API (as of version 1.1) that may impact our ability to (1) use it for all crypto operations in TLS and X.509 and (2) support isolation of all long-term secrets in TLS (that is, goals G1 and G2 in strategy.md in the same directory).

This is supposed to be a complete list, based on a exhaustive review of crypto operations done in TLS and X.509 code, but of course it's still possible that subtle-but-important issues have been missed. The only way to be really sure is, of course, to actually do the migration work.

Limitations relevant for G1 (performing crypto operations)

Restartable (aka interruptible) ECC operations

Support for interruptible ECDSA sign/verify was added to PSA in Mbed TLS 3.4. However, support for interruptible ECDH is not present yet. Also, PK, X.509 and TLS have not yet been adapted to take advantage of the new PSA APIs. See:

• https://github.com/Mbed-TLS/mbedtls/issues/7292;
• https://github.com/Mbed-TLS/mbedtls/issues/7293;
• https://github.com/Mbed-TLS/mbedtls/issues/7294.

Currently, when MBEDTLS_USE_PSA_CRYPTO and MBEDTLS_ECP_RESTARTABLE are both enabled, some operations that should be restartable are not (ECDH in TLS 1.2 clients using ECDHE-ECDSA), as they are using PSA instead, and some operations that should use PSA do not (signature generation & verification) as they use the legacy API instead, in order to get restartable behaviour.

Things that are in the API but not implemented yet

PSA Crypto has an API for FFDH, but it's not implemented in Mbed TLS yet. (Regarding FFDH, see the next section as well.) See issue 3261 on github.

Arbitrary parameters for FFDH

(See also the first paragraph in the previous section.)

Currently, the PSA Crypto API can only perform FFDH with a limited set of well-known parameters (some of them defined in the spec, but implementations are free to extend that set).

TLS 1.2 (and earlier) on the other hand have the server send explicit parameters (P and G) in its ServerKeyExchange message. This has been found to be suboptimal for security, as it is prohibitively hard for the client to verify the strength of these parameters. This led to the development of RFC 7919 which allows use of named groups in TLS 1.2 - however as this is only an extension, servers can still send custom parameters if they don't support the extension.

In TLS 1.3 the situation will be simpler: named groups are the only option, so the current PSA Crypto API is a good match for that. (Not coincidentally, all the groups used by RFC 7919 and TLS 1.3 are included in the PSA specification.)

There are several options here:

1. Implement support for custom FFDH parameters in PSA Crypto: this would pose non-trivial API design problem, but most importantly seems backwards, as the crypto community is moving away from custom FFDH parameters. (Could be done any time.)
2. Drop the DHE-RSA and DHE-PSK key exchanges in TLS 1.2 when moving to PSA. (For people who want some algorithmic variety in case ECC collapses, FFDH would still be available in TLS 1.3, just not in 1.2.) (Can only be done in 4.0 or another major version.)
3. Variant of the precedent: only drop client-side support. Server-side is easy to support in terms of API/protocol, as the server picks the parameters: we just need remove the existing mbedtls_ssl_conf_dh_param_xxx() APIs and tell people to use mbedtls_ssl_conf_groups() instead. (Can only be done in 4.0 or another major version.)
4. Implement RFC 7919, support DHE-RSA and DHE-PSK only in conjunction with it when moving to PSA. Server-side would work as above; unfortunately client-side the only option is to offer named groups and break the handshake if the server didn't take on our offer. This is not fully satisfying, but is perhaps the least unsatisfying option in terms of result; it's also probably the one that requires the most work, but it would deliver value beyond PSA migration by implementing RFC 7919. (Implementing RFC 7919 could be done any time; making it mandatory can only be done in 4.0 or another major version.)

As of early 2023, the plan is to go with option 2 in Mbed TLS 4.0, which has been announced on the mailing-list and got no push-back, see https://github.com/Mbed-TLS/mbedtls/issues/5278.

RSA-PSS parameters

RSA-PSS signatures are defined by PKCS#1 v2, re-published as RFC 8017 (previously RFC 3447).

As standardized, the signature scheme takes several parameters, in addition to the hash algorithm potentially used to hash the message being signed:

• a hash algorithm used for the encoding function
• a mask generation function
  • most commonly MGF1, which in turn is parametrized by a hash algorithm
• a salt length
• a trailer field - the value is fixed to 0xBC by PKCS#1 v2.1, but was left configurable in the original scheme; 0xBC is used everywhere in practice.

Both the existing mbedtls_ API and the PSA API support only MGF1 as the generation function (and only 0xBC as the trailer field), but there are discrepancies in handling the salt length and which of the various hash algorithms can differ from each other.

API comparison

• RSA:
  • signature: mbedtls_rsa_rsassa_pss_sign()
    • message hashed externally
    • encoding hash = MGF1 hash (from context, or argument = message hash)
    • salt length: always using the maximum legal value
  • signature: mbedtls_rsa_rsassa_pss_sign_ext()
    • message hashed externally
    • encoding hash = MGF1 hash (from context, or argument = message hash)
    • salt length: specified explicitly
  • verification: mbedtls_rsassa_pss_verify()
    • message hashed externally
    • encoding hash = MGF1 hash (from context, or argument = message hash)
    • salt length: any valid length accepted
  • verification: mbedtls_rsassa_pss_verify_ext()
    • message hashed externally
    • encoding hash = MGF1 hash from dedicated argument
    • expected salt length: specified explicitly, can specify "ANY"
• PK:
  • signature: not supported
  • verification: mbedtls_pk_verify_ext()
    • message hashed externally
    • encoding hash = MGF1 hash, specified explicitly
    • expected salt length: specified explicitly, can specify "ANY"
• PSA:
  • algorithm specification:
    • hash alg used for message hashing, encoding and MGF1
    • salt length can be either "standard" (<= hashlen, see note) or "any"
  • signature generation:
    • salt length: always <= hashlen (see note) and random salt
  • verification:
    • salt length: either <= hashlen (see note), or any depending on algorithm

Note: above, "<= hashlen" means that hashlen is used if possible, but if it doesn't fit because the key is too short, then the maximum length that fits is used.

The RSA/PK API is in principle more flexible than the PSA Crypto API. The following sub-sections study whether and how this matters in practice.

Use in X.509

RFC 4055 Section 3.1 defines the encoding of RSA-PSS that's used in X.509. It allows independently specifying the message hash (also used for encoding hash), the MGF (and its hash if MGF1 is used), and the salt length (plus an extra parameter "trailer field" that doesn't vary in practice"). These can be encoded as part of the key, and of the signature. If both encoding are presents, all values must match except possibly for the salt length, where the value from the signature parameters is used.

In Mbed TLS, RSA-PSS parameters can be parsed and displayed for various objects (certificates, CRLs, CSRs). During parsing, the following properties are enforced:

• the extra "trailer field" parameter must have its default value
• the mask generation function is MGF1
• encoding hash = message hashing algorithm (may differ from MGF1 hash)

When it comes to cryptographic operations, only two things are supported:

• verifying the signature on a certificate from its parent;
• verifying the signature on a CRL from the issuing CA.

The verification is done using mbedtls_pk_verify_ext().

Note: since X.509 parsing ensures that message hash = encoding hash, and mbedtls_pk_verify_ext() uses encoding hash = mgf1 hash, it looks like all three hash algorithms must be equal, which would be good news as it would match a limitation of the PSA API.

It is unclear what parameters people use in practice. It looks like by default OpenSSL picks saltlen = keylen - hashlen - 2 (tested with openssl 1.1.1f). The certtool command provided by GnuTLS seems to be picking saltlen = hashlen by default (tested with GnuTLS 3.6.13). FIPS 186-4 requires 0 <= saltlen <= hashlen.

Use in TLS

In TLS 1.2 (or lower), RSA-PSS signatures are never used, except via X.509.

In TLS 1.3, RSA-PSS signatures can be used directly in the protocol (in addition to indirect use via X.509). It has two sets of three signature algorithm identifiers (for SHA-256, SHA-384 and SHA-512), depending of what the OID of the public key is (rsaEncryption or RSASSA-PSS).

In both cases, it specifies that:

• the mask generation function is MGF1
• all three hashes are equal
• the length of the salt MUST be equal to the length of the digest algorithm

When signing, the salt length picked by PSA is the one required by TLS 1.3 (unless the key is unreasonably small).

When verifying signatures, PSA will by default enforce the salt len is the one required by TLS 1.3.

Current testing - X509

All test files use the default trailer field of 0xBC, as enforced by our parser. (There's a negative test for that using the x509_parse_rsassa_pss_params test function and hex data.)

Files with "bad" in the name are expected to be invalid and rejected in tests.

Test certificates:

server9-bad-mgfhash.crt (announcing mgf1(sha224), signed with another mgf) Hash Algorithm: sha256 Mask Algorithm: mgf1 with sha224 Salt Length: 0xDE server9-bad-saltlen.crt (announcing saltlen = 0xDE, signed with another len) Hash Algorithm: sha256 Mask Algorithm: mgf1 with sha256 Salt Length: 0xDE server9-badsign.crt (one bit flipped in the signature) Hash Algorithm: sha1 (default) Mask Algorithm: mgf1 with sha1 (default) Salt Length: 0xEA server9-defaults.crt Hash Algorithm: sha1 (default) Mask Algorithm: mgf1 with sha1 (default) Salt Length: 0x14 (default) server9-sha224.crt Hash Algorithm: sha224 Mask Algorithm: mgf1 with sha224 Salt Length: 0xE2 server9-sha256.crt Hash Algorithm: sha256 Mask Algorithm: mgf1 with sha256 Salt Length: 0xDE server9-sha384.crt Hash Algorithm: sha384 Mask Algorithm: mgf1 with sha384 Salt Length: 0xCE server9-sha512.crt Hash Algorithm: sha512 Mask Algorithm: mgf1 with sha512 Salt Length: 0xBE server9-with-ca.crt Hash Algorithm: sha1 (default) Mask Algorithm: mgf1 with sha1 (default) Salt Length: 0xEA server9.crt Hash Algorithm: sha1 (default) Mask Algorithm: mgf1 with sha1 (default) Salt Length: 0xEA

These certificates are signed with a 2048-bit key. It appears that they are all using saltlen = keylen - hashlen - 2, except for server9-defaults which is using saltlen = hashlen.

Test CRLs:

crl-rsa-pss-sha1-badsign.pem Hash Algorithm: sha1 (default) Mask Algorithm: mgf1 with sha1 (default) Salt Length: 0xEA crl-rsa-pss-sha1.pem Hash Algorithm: sha1 (default) Mask Algorithm: mgf1 with sha1 (default) Salt Length: 0xEA crl-rsa-pss-sha224.pem Hash Algorithm: sha224 Mask Algorithm: mgf1 with sha224 Salt Length: 0xE2 crl-rsa-pss-sha256.pem Hash Algorithm: sha256 Mask Algorithm: mgf1 with sha256 Salt Length: 0xDE crl-rsa-pss-sha384.pem Hash Algorithm: sha384 Mask Algorithm: mgf1 with sha384 Salt Length: 0xCE crl-rsa-pss-sha512.pem Hash Algorithm: sha512 Mask Algorithm: mgf1 with sha512 Salt Length: 0xBE

These CRLs are signed with a 2048-bit key. It appears that they are all using saltlen = keylen - hashlen - 2.

Test CSRs:

server9.req.sha1 Hash Algorithm: sha1 (default) Mask Algorithm: mgf1 with sha1 (default) Salt Length: 0x6A server9.req.sha224 Hash Algorithm: sha224 Mask Algorithm: mgf1 with sha224 Salt Length: 0x62 server9.req.sha256 Hash Algorithm: sha256 Mask Algorithm: mgf1 with sha256 Salt Length: 0x5E server9.req.sha384 Hash Algorithm: sha384 Mask Algorithm: mgf1 with sha384 Salt Length: 0x4E server9.req.sha512 Hash Algorithm: sha512 Mask Algorithm: mgf1 with sha512 Salt Length: 0x3E

These CSRs are signed with a 2048-bit key. It appears that they are all using saltlen = keylen - hashlen - 2.

Possible courses of action

There's no question about what to do with TLS (any version); the only question is about X.509 signature verification. Options include:

1. Doing all verifications with PSA_ALG_RSA_PSS_ANY_SALT - while this wouldn't cause a concrete security issue, this would be non-compliant.
2. Doing verifications with PSA_ALG_RSA_PSS when we're lucky and the encoded saltlen happens to match hashlen, and falling back to ANY_SALT otherwise. Same issue as with the previous point, except more contained.
3. Reject all certificates with saltlen != hashlen. This includes all certificates generated with OpenSSL using the default parameters, so it's probably not acceptable.
4. Request an extension to the PSA Crypto API and use one of the above options in the meantime. Such an extension seems inconvenient and not motivated by strong security arguments, so it's unclear whether it would be accepted.

Since Mbed TLS 3.4, option 1 is implemented.

Limitations relevant for G2 (isolation of long-term secrets)

Currently none.