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| 1 | +// Copyright 2019 Contributors to the Parsec project. |
| 2 | +// SPDX-License-Identifier: Apache-2.0 |
| 3 | + |
| 4 | +use aes::cipher::AsyncStreamCipher; |
| 5 | +use cfb_mode::cipher::BlockCipherEncrypt; |
| 6 | +use core::{ |
| 7 | + marker::PhantomData, |
| 8 | + ops::{Add, Mul}, |
| 9 | +}; |
| 10 | +use digest::{ |
| 11 | + array::ArraySize, |
| 12 | + consts::{B1, U8}, |
| 13 | + crypto_common::{Iv, KeyIvInit, KeySizeUser}, |
| 14 | + typenum::{ |
| 15 | + operator_aliases::{Add1, Sum}, |
| 16 | + Unsigned, |
| 17 | + }, |
| 18 | + Digest, FixedOutputReset, KeyInit, Mac, OutputSizeUser, |
| 19 | +}; |
| 20 | +use ecdsa::elliptic_curve::{ |
| 21 | + ecdh::{EphemeralSecret, SharedSecret}, |
| 22 | + sec1::{FromEncodedPoint, ModulusSize, ToEncodedPoint}, |
| 23 | + AffinePoint, Curve, CurveArithmetic, FieldBytesSize, PublicKey, |
| 24 | +}; |
| 25 | +use hmac::{EagerHash, Hmac}; |
| 26 | +use rand::thread_rng; |
| 27 | + |
| 28 | +use crate::{ |
| 29 | + structures::{EncryptedSecret, IdObject, Name}, |
| 30 | + utils::kdf::{self}, |
| 31 | +}; |
| 32 | + |
| 33 | +// TpmHmacKey intends to code for the key expected for hmac |
| 34 | +// in the KDFa derivations. There are no standard sizes for hmac keys really, |
| 35 | +// upstream RustCrypto considers it to be BlockSize, but TPM specification |
| 36 | +// has a different opinion on the matter, and expect the key to the output |
| 37 | +// bit size of the hash algorithm used. |
| 38 | +// |
| 39 | +// See https://trustedcomputinggroup.org/wp-content/uploads/TPM-2.0-1.83-Part-1-Architecture.pdf#page=202 |
| 40 | +// section 24.5 HMAC: |
| 41 | +// bits the number of bits in the digest produced by ekNameAlg |
| 42 | +struct TpmHmac<H>(PhantomData<H>); |
| 43 | + |
| 44 | +impl<H> KeySizeUser for TpmHmac<H> |
| 45 | +where |
| 46 | + H: OutputSizeUser, |
| 47 | +{ |
| 48 | + type KeySize = H::OutputSize; |
| 49 | +} |
| 50 | + |
| 51 | +pub fn make_credential_ecc<C, EkHash, EkCipher>( |
| 52 | + ek_public: PublicKey<C>, |
| 53 | + secret: &[u8], |
| 54 | + key_name: Name, |
| 55 | +) -> (IdObject, EncryptedSecret) |
| 56 | +where |
| 57 | + C: Curve + CurveArithmetic, |
| 58 | + |
| 59 | + AffinePoint<C>: FromEncodedPoint<C> + ToEncodedPoint<C>, |
| 60 | + FieldBytesSize<C>: ModulusSize, |
| 61 | + |
| 62 | + <FieldBytesSize<C> as Add>::Output: Add<FieldBytesSize<C>>, |
| 63 | + Sum<FieldBytesSize<C>, FieldBytesSize<C>>: ArraySize, |
| 64 | + Sum<FieldBytesSize<C>, FieldBytesSize<C>>: Add<U8>, |
| 65 | + Sum<Sum<FieldBytesSize<C>, FieldBytesSize<C>>, U8>: Add<B1>, |
| 66 | + Add1<Sum<Sum<FieldBytesSize<C>, FieldBytesSize<C>>, U8>>: ArraySize, |
| 67 | + |
| 68 | + EkHash: Digest + EagerHash + FixedOutputReset, |
| 69 | + <EkHash as OutputSizeUser>::OutputSize: Mul<U8>, |
| 70 | + <<EkHash as OutputSizeUser>::OutputSize as Mul<U8>>::Output: Unsigned, |
| 71 | + <<EkHash as EagerHash>::Core as OutputSizeUser>::OutputSize: ArraySize + Mul<U8>, |
| 72 | + <<<EkHash as EagerHash>::Core as OutputSizeUser>::OutputSize as Mul<U8>>::Output: Unsigned, |
| 73 | + |
| 74 | + EkCipher: KeySizeUser + BlockCipherEncrypt + KeyInit, |
| 75 | + <EkCipher as KeySizeUser>::KeySize: Mul<U8>, |
| 76 | + <<EkCipher as KeySizeUser>::KeySize as Mul<U8>>::Output: ArraySize, |
| 77 | +{ |
| 78 | + let mut rng = thread_rng(); |
| 79 | + |
| 80 | + // See Table 22 - Key Generation for the various labels used here after: |
| 81 | + // https://trustedcomputinggroup.org/wp-content/uploads/TPM-2.0-1.83-Part-1-Architecture.pdf#page=183 |
| 82 | + |
| 83 | + // C.6.4. ECC Secret Sharing for Credentials |
| 84 | + // https://trustedcomputinggroup.org/wp-content/uploads/TPM-2.0-1.83-Part-1-Architecture.pdf#page=311 |
| 85 | + let local = EphemeralSecret::<C>::random(&mut rng); |
| 86 | + |
| 87 | + let ecdh_secret: SharedSecret<C> = local.diffie_hellman(&ek_public); |
| 88 | + let local_public = local.public_key(); |
| 89 | + drop(local); |
| 90 | + |
| 91 | + let seed = kdf::kdfe::<kdf::Identity, EkHash, C, TpmHmac<EkHash>>( |
| 92 | + &ecdh_secret, |
| 93 | + &local_public, |
| 94 | + &ek_public, |
| 95 | + ); |
| 96 | + drop(ecdh_secret); |
| 97 | + |
| 98 | + // The local ECDH pair is used as "encrypted seed" |
| 99 | + let encrypted_seed = { |
| 100 | + let mut out = vec![]; |
| 101 | + out.extend_from_slice(&FieldBytesSize::<C>::U16.to_be_bytes()[..]); |
| 102 | + out.extend_from_slice(&local_public.to_encoded_point(false).x().unwrap()); |
| 103 | + out.extend_from_slice(&FieldBytesSize::<C>::U16.to_be_bytes()[..]); |
| 104 | + out.extend_from_slice(&local_public.to_encoded_point(false).y().unwrap()); |
| 105 | + out |
| 106 | + }; |
| 107 | + |
| 108 | + // Prepare the sensitive data |
| 109 | + // this will be then encrypted using AES-CFB (size of the symmetric key depends on the EK). |
| 110 | + let mut sensitive_data = { |
| 111 | + let mut out = vec![]; |
| 112 | + out.extend_from_slice(&u16::try_from(secret.len()).unwrap().to_be_bytes()[..]); |
| 113 | + out.extend_from_slice(secret); |
| 114 | + out |
| 115 | + }; |
| 116 | + |
| 117 | + // We'll now encrypt the sensitive data, and hmac the result of the encryption |
| 118 | + // https://trustedcomputinggroup.org/wp-content/uploads/TPM-2.0-1.83-Part-1-Architecture.pdf#page=201 |
| 119 | + // See 24.4 Symmetric Encryption |
| 120 | + let sym_key = kdf::kdfa::<EkHash, kdf::Storage, EkCipher>(&seed, key_name.value(), &[]); |
| 121 | + |
| 122 | + // TODO(baloo): once the weak key detection merges |
| 123 | + // https://github.com/RustCrypto/traits/pull/1739 |
| 124 | + // https://github.com/RustCrypto/block-ciphers/pull/465 |
| 125 | + // |
| 126 | + // We should check for weak keys, and re-run all the steps above until we get a non-weak key |
| 127 | + // this is to be in compliance with TPM spec section 11.4.10.4: |
| 128 | + // https://trustedcomputinggroup.org/wp-content/uploads/TPM-2.0-1.83-Part-1-Architecture.pdf#page=82 |
| 129 | + |
| 130 | + let iv: Iv<cfb_mode::Encryptor<EkCipher>> = Default::default(); |
| 131 | + |
| 132 | + cfb_mode::Encryptor::<EkCipher>::new(&sym_key.into(), &iv.into()).encrypt(&mut sensitive_data); |
| 133 | + |
| 134 | + // See 24.5 HMAC |
| 135 | + let hmac_key = kdf::kdfa::<EkHash, kdf::Integrity, TpmHmac<EkHash>>(&seed, &[], &[]); |
| 136 | + let mut hmac = Hmac::<EkHash>::new_from_slice(&hmac_key).unwrap(); |
| 137 | + hmac.update(&sensitive_data); |
| 138 | + hmac.update(key_name.value()); |
| 139 | + let hmac = hmac.finalize(); |
| 140 | + |
| 141 | + // We'll now serialize the object and get everything through the door. |
| 142 | + let mut out = vec![]; |
| 143 | + out.extend_from_slice( |
| 144 | + &u16::try_from(hmac.into_bytes().len()) |
| 145 | + .unwrap() |
| 146 | + .to_be_bytes()[..], |
| 147 | + ); |
| 148 | + out.extend_from_slice(&hmac.into_bytes()); |
| 149 | + out.extend_from_slice(&sensitive_data); |
| 150 | + |
| 151 | + ( |
| 152 | + IdObject::from_bytes(&out).unwrap(), |
| 153 | + EncryptedSecret::from_bytes(&encrypted_seed).unwrap(), |
| 154 | + ) |
| 155 | +} |
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