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# ECC Asymmetric-Key Algorithm

ECC is another type of asymmetric mathematics that is used for cryptography. Unlike RSA, which uses an easily understood mathematical operation—factoring a product of two large primes—ECC uses more difficult mathematical concepts based on elliptic curves over a finite field. We will not describe the mathematics but instead describe how it is used. Just like every other asymmetric algorithm, ECC has a private and public key pair. The public key can be used to verify something signed with the private key, and the private key can be used to decrypt data that was encrypted using the public key.

For equivalent strength, ECC keys are much smaller than RSA keys. The strength of an ECC key is half the key size, so a 256-bit ECC key has 128 bits of strength. A similarly strong RSA key is 3,076 bits long. Smaller key sizes use fewer resources and perform faster. For encryption, a procedure known as Elliptic Curve Diffie-Hellman (ECDH) is used with ECC. For signing, Elliptic Curve Digital Signature Algorithm (ECDSA) is used.

# ECDH Asymmetric-Key Algorithm to Use Elliptic Curves to Pass Keys

When using ECC to encrypt/decrypt asymmetrically, you use the ECDH algorithm. The main difference between ECC and RSA for encryption/decryption is that the process of using an ECDH key takes two steps, whereas RSA takes only one. When encrypting a symmetric key with a TPM-based RSA key, you use the TPM RSA's public key to encrypt it. When encrypting a symmetric key with a TPM-based ECDH key, two steps are required: Generate (in software) another ECDH key; and then use the private key of the newly generated ECDH key and the public portion of the TPM ECDH key to generate a new ephemeral random number, which is input to a KDF to generate a symmetric key. To put this more succinctly, with RSA you can supply the symmetric key to be encrypted, but with ECDH the process generates the symmetric key.

To recover the symmetric key, the public portion of the software-generated ECDH key is given to the TPM. It uses it together with the private portion of its own ECDH key to regenerate the ephemeral random number, which it inputs into a KDF internally to regenerate the symmetric key.

# ECDSA Asymmetric-Key Algorithm to Use Elliptic Curves for Signatures

ECDSA is used as an algorithm with ECC to produce signatures. Just as with RSA, in ECDSA the private key is used to sign and the public key is used to verify the signature. The main difference (other than the mathematical steps used) is that when using an ECC key, because it's much smaller than an RSA key, you have to ensure that the hash of the message you're signing isn't too big. The ECDSA signature signs only n bits of the hash, where n is the size of the key. (This is also true of RSA; but RSA keys sizes are typically >=1,024 bits and hash sizes top out at 512 bits, so this is never a problem.)

Whereas with RSA you can typically sign a message with any hash algorithm, with ECC you typically use a hash algorithm that matches the size of the key: SHA-256 for ECC-256 and SHA-384 for ECC-384. If you used SHA-512 (which produces 512-bit hashes) with an ECC-384 key, ECDSA would sign only the first 384 bits of the hash. You can sign smaller hashes without any problem, of course, so an ECC 384-bit key could be used to sign SHA-384, SHA-256, or SHA-1 (160 bits) hashes.

One problem with all signing protocols is that the recipient of the signature needs to be assured that the public key they use to verify the signature really belongs to the owner of the private key who signed it. This is handled with public key certificates.

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