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Question about the ECDH

JavaCard Applet Development Related Questions and Answers.
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Question about the ECDH

Post by iSmartcard » Sun Jul 08, 2018 3:13 am

Hello everyone, when I use the bouncycastle encryption library, I get a question about ECDH. With ECDH, the length of the value is 32 bytes and the length of the javacard is 20 bytes. What happened? Does anyone know why?

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Re: Question about the ECDH

Post by AlexLiu » Mon Jul 09, 2018 5:43 am

32 bytes corresponding to ECC256, ECDH protocol based on ecc curve 256 bit,
20 bytes corresponding to ECC160, ECDH protocol based on ecc curve 160 bit

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Re: Question about the ECDH

Post by scplatform » Mon Jul 09, 2018 6:07 am

According the API specification of jc304 or above, the result will be hash with SHA1.
public static final byte ALG_EC_SVDP_DH
Elliptic curve secret value derivation primitive, Diffie-Hellman version, as per [IEEE P1363].

This algorithm computes the SHA-1 message digest of the output of the derivation primitive to yield a 20 byte result.
See Also:
Constant Field Values

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Re: Question about the ECDH

Post by happy123654 » Fri Jul 13, 2018 5:21 am

there will have SHA_1 calculation in the last step of ECDH and the result of SHA_1 is 20 bytes

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