aws_lc_rs

Module signature

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Public key signatures: signing and verification.

Use the verify function to verify signatures, passing a reference to the algorithm that identifies the algorithm. See the documentation for verify for examples.

For signature verification, this API treats each combination of parameters as a separate algorithm. For example, instead of having a single “RSA” algorithm with a verification function that takes a bunch of parameters, there are RSA_PKCS1_2048_8192_SHA256, RSA_PKCS1_2048_8192_SHA384, etc., which encode sets of parameter choices into objects. This is designed to reduce the risks of algorithm agility and to provide consistency with ECDSA and EdDSA.

Currently this module does not support digesting the message to be signed separately from the public key operation, as it is currently being optimized for Ed25519 and for the implementation of protocols that do not requiring signing large messages. An interface for efficiently supporting larger messages may be added later.

§Algorithm Details

§ECDSA_*_ASN1 Details: ASN.1-encoded ECDSA Signatures

The signature is a ASN.1 DER-encoded Ecdsa-Sig-Value as described in RFC 3279 Section 2.2.3. This is the form of ECDSA signature used in X.509-related structures and in TLS’s ServerKeyExchange messages.

The public key is encoding in uncompressed form using the Octet-String-to-Elliptic-Curve-Point algorithm in SEC 1: Elliptic Curve Cryptography, Version 2.0.

During verification, the public key is validated using the ECC Partial Public-Key Validation Routine from Section 5.6.2.3.3 of NIST Special Publication 800-56A, revision 2 and Appendix A.3 of the NSA’s Suite B implementer’s guide to FIPS 186-3. Note that, as explained in the NSA guide, ECC Partial Public-Key Validation is equivalent to ECC Full Public-Key Validation for prime-order curves like this one.

§ECDSA_*_FIXED Details: Fixed-length (PKCS#11-style) ECDSA Signatures

The signature is r||s, where || denotes concatenation, and where both r and s are both big-endian-encoded values that are left-padded to the maximum length. A P-256 signature will be 64 bytes long (two 32-byte components) and a P-384 signature will be 96 bytes long (two 48-byte components). This is the form of ECDSA signature used PKCS#11 and DNSSEC.

The public key is encoding in uncompressed form using the Octet-String-to-Elliptic-Curve-Point algorithm in SEC 1: Elliptic Curve Cryptography, Version 2.0.

During verification, the public key is validated using the ECC Partial Public-Key Validation Routine from Section 5.6.2.3.3 of NIST Special Publication 800-56A, revision 2 and Appendix A.3 of the NSA’s Suite B implementer’s guide to FIPS 186-3. Note that, as explained in the NSA guide, ECC Partial Public-Key Validation is equivalent to ECC Full Public-Key Validation for prime-order curves like this one.

§RSA_PKCS1_* Details: RSA PKCS#1 1.5 Signatures

The signature is an RSASSA-PKCS1-v1_5 signature as described in RFC 3447 Section 8.2.

The public key is encoded as an ASN.1 RSAPublicKey as described in RFC 3447 Appendix-A.1.1. The public key modulus length, rounded up to the nearest (larger) multiple of 8 bits, must be in the range given in the name of the algorithm. The public exponent must be an odd integer of 2-33 bits, inclusive.

§RSA_PSS_* Details: RSA PSS Signatures

The signature is an RSASSA-PSS signature as described in RFC 3447 Section 8.1.

The public key is encoded as an ASN.1 RSAPublicKey as described in RFC 3447 Appendix-A.1.1. The public key modulus length, rounded up to the nearest (larger) multiple of 8 bits, must be in the range given in the name of the algorithm. The public exponent must be an odd integer of 2-33 bits, inclusive.

During verification, signatures will only be accepted if the MGF1 digest algorithm is the same as the message digest algorithm and if the salt length is the same length as the message digest. This matches the requirements in TLS 1.3 and other recent specifications.

During signing, the message digest algorithm will be used as the MGF1 digest algorithm. The salt will be the same length as the message digest. This matches the requirements in TLS 1.3 and other recent specifications. Additionally, the entire salt is randomly generated separately for each signature using the secure random number generator passed to sign().

§Examples

§Signing and verifying with Ed25519

use aws_lc_rs::{
    rand,
    signature::{self, KeyPair},
};

fn main() -> Result<(), aws_lc_rs::error::Unspecified> {
    // Generate a key pair in PKCS#8 (v1) format.
    let rng = rand::SystemRandom::new();
    let pkcs8_bytes = signature::Ed25519KeyPair::generate_pkcs8v1(&rng)?;

    // Normally the application would store the PKCS#8 file persistently. Later
    // it would read the PKCS#8 file from persistent storage to use it.

    let key_pair = signature::Ed25519KeyPair::from_pkcs8_maybe_unchecked(pkcs8_bytes.as_ref())?;

    // Sign the message "hello, world".
    const MESSAGE: &[u8] = b"hello, world";
    let sig = key_pair.sign(MESSAGE);

    // Normally an application would extract the bytes of the signature and
    // send them in a protocol message to the peer(s). Here we just get the
    // public key key directly from the key pair.
    let peer_public_key_bytes = key_pair.public_key().as_ref();

    // Verify the signature of the message using the public key. Normally the
    // verifier of the message would parse the inputs to this code out of the
    // protocol message(s) sent by the signer.
    let peer_public_key =
        signature::UnparsedPublicKey::new(&signature::ED25519, peer_public_key_bytes);
    peer_public_key.verify(MESSAGE, sig.as_ref())?;

    Ok(())
}

§Signing and verifying with RSA (PKCS#1 1.5 padding)

By default OpenSSL writes RSA public keys in SubjectPublicKeyInfo format, not RSAPublicKey format, and Base64-encodes them (“PEM” format).

To convert the PEM SubjectPublicKeyInfo format (“BEGIN PUBLIC KEY”) to the binary RSAPublicKey format needed by verify(), use:

openssl rsa -pubin \
            -in public_key.pem \
            -inform PEM \
            -RSAPublicKey_out \
            -outform DER \
            -out public_key.der

To extract the RSAPublicKey-formatted public key from an ASN.1 (binary) DER-encoded RSAPrivateKey format private key file, use:

openssl rsa -in private_key.der \
            -inform DER \
            -RSAPublicKey_out \
            -outform DER \
            -out public_key.der
use aws_lc_rs::{rand, signature};

fn sign_and_verify_rsa(
    private_key_path: &std::path::Path,
    public_key_path: &std::path::Path,
) -> Result<(), MyError> {
    // Create an `RsaKeyPair` from the DER-encoded bytes. This example uses
    // a 2048-bit key, but larger keys are also supported.
    let private_key_der = read_file(private_key_path)?;
    let key_pair = signature::RsaKeyPair::from_der(&private_key_der)
        .map_err(|_| MyError::BadPrivateKey)?;

    // Sign the message "hello, world", using PKCS#1 v1.5 padding and the
    // SHA256 digest algorithm.
    const MESSAGE: &'static [u8] = b"hello, world";
    let rng = rand::SystemRandom::new();
    let mut signature = vec![0; key_pair.public_modulus_len()];
    key_pair
        .sign(&signature::RSA_PKCS1_SHA256, &rng, MESSAGE, &mut signature)
        .map_err(|_| MyError::OOM)?;

    // Verify the signature.
    let public_key = signature::UnparsedPublicKey::new(
        &signature::RSA_PKCS1_2048_8192_SHA256,
        read_file(public_key_path)?,
    );
    public_key
        .verify(MESSAGE, &signature)
        .map_err(|_| MyError::BadSignature)
}

#[derive(Debug)]
enum MyError {
    IO(std::io::Error),
    BadPrivateKey,
    OOM,
    BadSignature,
}

fn read_file(path: &std::path::Path) -> Result<Vec<u8>, MyError> {
    use std::io::Read;

    let mut file = std::fs::File::open(path).map_err(|e| MyError::IO(e))?;
    let mut contents: Vec<u8> = Vec::new();
    file.read_to_end(&mut contents)
        .map_err(|e| MyError::IO(e))?;
    Ok(contents)
}

fn main() {
    let private_key_path =
        std::path::Path::new("tests/data/signature_rsa_example_private_key.der");
    let public_key_path =
        std::path::Path::new("tests/data/signature_rsa_example_public_key.der");
    sign_and_verify_rsa(&private_key_path, &public_key_path).unwrap()
}

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