pw_crypto#

SHA256#

1. Obtaining a oneshot digest.

#include "pw_crypto/sha256.h"

std::byte digest[32];
if (!pw::crypto::sha256::Hash(message, digest).ok()) {
  // Handle errors.
}

// The content can also come from a pw::stream::Reader.
if (!pw::crypto::sha256::Hash(reader, digest).ok()) {
  // Handle errors.
}

2. Hashing a long, potentially non-contiguous message.

#include "pw_crypto/sha256.h"

std::byte digest[32];

if (!pw::crypto::sha256::Sha256()
    .Update(chunk1).Update(chunk2).Update(chunk...)
    .Final().ok()) {
  // Handle errors.
}

ECDSA#

1. Verifying a digital signature signed with ECDSA over the NIST P256 curve.

#include "pw_crypto/sha256.h"

std::byte digest[32];
if (!pw::crypto::sha256::Hash(message, digest).ok()) {
  // handle errors.
}

if (!pw::crypto::ecdsa::VerifyP256Signature(public_key, digest,
                                            signature).ok()) {
  // handle errors.
}

2. Verifying a digital signature signed with ECDSA over the NIST P256 curve, with a long and/or non-contiguous message.

#include "pw_crypto/sha256.h"

std::byte digest[32];

if (!pw::crypto::sha256::Sha256()
    .Update(chunk1).Update(chunk2).Update(chunkN)
    .Final(digest).ok()) {
    // Handle errors.
}

if (!pw::crypto::ecdsa::VerifyP256Signature(public_key, digest,
                                            signature).ok()) {
    // Handle errors.
}

AES#

1. Computing the AES-CMAC of a potentially long and/or non-contiguous message. This is similar to a hash or digest except that the operation takes a secret key as an input, so the MAC can be used to verify integrity and authentication.

#include "pw_crypto/aes.h"

std::byte mac[16];

if (!pw::crypto::aes_cmac::Cmac(key).Update(chunk1).Update(chunk2)
      .Update(chunk...).Final().ok()) {
  // Handle errors.
}

2. Encrypting a single AES 128-bit block.

#include "pw_crypto/aes.h"

std::byte encrypted[16];

if (!pw::crypto::unsafe::aes::EncryptBlock(key, message, encrypted).ok()) {
    // Handle errors.
}

ECDH#

1. Generating a keypair and computing a shared symmetric key.

#include "pw_crypto/ecdh.h"

// Import the public key from the other party.
PW_TRY_ASSIGN(
   auto public_key,
   pw::crypto::ecdh::P256PublicKey::Import(other_x, other_y, endian));
PW_TRY_ASSIGN(auto keypair,
              pw::crypto::ecdh::P256Keypair::Generate());

std::byte shared_key[32];
if (!keypair.ComputeDiffieHellman(public_key, shared_key)) {
   // handle errors.
}

2. Import a pre-existing keypair (for testing purposes) and computing a shared symmetric key.

#include "pw_crypto/ecdh.h"

// Import the public key from the other party.
PW_TRY_ASSIGN(
   auto public_key,
   pw::crypto::ecdh::P256PublicKey::Import(other_x, other_y, endian));
PW_TRY_ASSIGN(auto keypair,
   pw::crypto::ecdh::P256Keypair::ImportForTesting(
      private_key, x, y, endian));

std::byte shared_key[32];
if (!keypair.ComputeDiffieHellman(public_key, shared_key)) {
   // handle errors.
}

Configuration#

The crypto services offered by pw_crypto can be backed by different backend crypto libraries.

# Install and configure MbedTLS
pw package install mbedtls
gn gen out --args='
    dir_pw_third_party_mbedtls=getenv("PW_PACKAGE_ROOT")+"/mbedtls"
    pw_crypto_SHA256_BACKEND="//pw_crypto:sha256_mbedtls_v3"
    pw_crypto_ECDSA_BACKEND="//pw_crypto:ecdsa_mbedtls_v3"
    pw_crypto_AES_BACKEND="//pw_crypto:aes_mbedtls_v3"
'

ninja -C out

platform(
  name = "my_platform",
  flags = [
     "@pigweed//pw_crypto:sha256_backend=@pigweed//pw_crypto:sha256_mbedtls_backend",
     "@pigweed//pw_crypto:ecdsa_backend=@pigweed//pw_crypto:ecdsa_mbedtls_backend",
     "@pigweed//pw_crypto:aes_backend=@pigweed//pw_crypto:aes_mbedtls_backend",
     # ... other flags
   ],
)

#define MBEDTLS_SHA256_SMALLER

#define MBEDTLS_BIGNUM_C
#define MBEDTLS_ECP_C
#define MBEDTLS_ECDSA_C
// The ASN1 options are needed only because mbedtls considers and verifies
// them (in check_config.h) as dependencies of MBEDTLS_ECDSA_C.
#define MBEDTLS_ASN1_WRITE_C
#define MBEDTLS_ASN1_PARSE_C
#define MBEDTLS_ECP_NO_INTERNAL_RNG
#define MBEDTLS_ECP_DP_SECP256R1_ENABLED

# Install and configure BoringSSL
pw package install boringssl
gn gen out --args='
    dir_pw_third_party_boringssl=getenv("PW_PACKAGE_ROOT")+"/boringssl"
    pw_crypto_AES_BACKEND="//pw_crypto:aes_boringssl"
    pw_crypto_ECDH_BACKEND="//pw_crypto:ecdh_boringssl"
'

ninja -C out

platform(
  name = "my_platform",
  constraint_values = [
    "@pigweed//pw_aes:aes_boringssl_backend",
    # ... other constraint_values
  ],
)

Size Reports#

Below are size reports for each crypto service. These vary across configurations.

API reference#

Status pw::crypto::ecdsa::VerifyP256Signature(ConstByteSpan public_key, ConstByteSpan digest, ConstByteSpan signature)#

Verifies the signature of digest using public_key.

Example:

#include "pw_crypto/sha256.h"

// Verify a digital signature signed with ECDSA over the NIST P256 curve.
std::byte digest[32];
if (!pw::crypto::sha256::Hash(message, digest).ok()) {
    // handle errors.
}

if (!pw::crypto::ecdsa::VerifyP256Signature(public_key, digest,
                                            signature).ok()) {
    // handle errors.
}

Parameters:

• public_key – [in] A byte string in SEC 1 uncompressed form (0x04||X||Y), which is exactly 65 bytes. Compressed forms (02/03||X) may not be supported by some backends, e.g. Mbed TLS.

• digest – [in] A raw byte string, truncated to 32 bytes.

• signature – [in] A raw byte string (r||s) of exactly 64 bytes.

Returns:

OK for a successful verification, or an error Status otherwise.

inline Status pw::crypto::sha256::Hash(ConstByteSpan message, ByteSpan out_digest)#

Calculates the SHA256 digest of message and stores the result in out_digest. out_digest must be at least kDigestSizeBytes long.

One-shot digest example:

#include "pw_crypto/sha256.h"

std::byte digest[32];
if (!pw::crypto::sha256::Hash(message, digest).ok()) {
    // Handle errors.
}

// The content can also come from a pw::stream::Reader.
if (!pw::crypto::sha256::Hash(reader, digest).ok()) {
    // Handle errors.
}

Long, potentially non-contiguous message example:

#include "pw_crypto/sha256.h"

std::byte digest[32];

if (!pw::crypto::sha256::Sha256()
    .Update(chunk1).Update(chunk2).Update(chunk...)
    .Final().ok()) {
    // Handle errors.
}

inline Status pw::crypto::sha256::Hash(stream::Reader &reader, ByteSpan out_digest)#

constexpr uint32_t pw::crypto::sha256::kDigestSizeBytes = 32#

The size of a SHA256 digest in bytes.

inline Status pw::crypto::sha256::Sha256::Final(ByteSpan out_digest)#

Finishes the hashing session and outputs the final digest in the first kDigestSizeBytes of out_digest. out_digest must be at least kDigestSizeBytes long.

Final() locks down the Sha256 instance from any additional use.

Any error, including those occurring inside the constructor or Update() will be reflected in the return value of Final().

inline Sha256 &pw::crypto::sha256::Sha256::Update(ConstByteSpan data)#

Feeds data to the running hasher. The feeding can involve zero or more Update() calls and the order matters.

enum class pw::crypto::sha256::Sha256State#

A state machine of a hashing session.

Values:

enumerator kReady#

Initialized and accepting input (via Update()).

enumerator kFinalized#

Finalized by Final(). Any additional requests to Update() or Final() will trigger a transition to kError.

enumerator kError#

In an unrecoverable error state.