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POCheck if the file is correct using sha1withRsa via openssl functions
 

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    <h2>Hi, i have file_data(xml format) and file_signature(ASN1 DER), and also have certificate(X509 ASN1 DER). I want to check if file_data is correct, but I have some problems. what I'm doing:</h2> <p>Main Idea: Some company A creates file_data, then using SHA1 gets hash of the file_data, and encrypts this hash using RSA private key and gets file_signature. Then company A sends me file_data and file_signature and certificate. I get public key from certificate get file_signature and decrypt file_signature using public key and get hash_1. Then i get file_data and use SHA1 to get hash_2. If hash_1 and hash_2 is equal, i can trust to content of the file_data, am I right? </p> <p>Implementation:</p> <ol> <li>Load certificate: <code>d2i_X509_fp()</code> function. Now I have certificate.</li> <li>Get public key of the certificate: <code>X509_extract_key</code>, now i have public key.</li> <li><strong>Now i want to load file_signature to decrypt it using public key, BUT file_signature has ASN1 DER format how I can load it, what function in OpenSSl should I use?</strong></li> <li>Suppose I read file_signature, now I must decrypt it using my public key, is there any API for this purpose?</li> <li>Suppose I decrypt file_signature and get hash_1.</li> <li>Now I must load file_data and get hash of it using SHA1 function hash_2, what function I must use? <code>SHA1()</code>, or <code>SHA1_Init, SHA1_Update, SHA1_Finish</code>?</li> <li>Suppose I get hash_1 and hash_2, how i must compare them, using <code>memcmp</code>?</li> <li>Suppose I compare them, if they are equal, i can use file_data.</li> </ol> <p>another question is that file_signature is 128 byte len and when i decrypt it i get 128 byte hash_1(Am I rigth) , but when i get hash of the file_data hash_2 it's length is only 20 bytes, so how I can compare them, or I misunderstand something?</p> <p>Thanks for your help! p.s. sorry for my english;). </p>
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    1. COASN1 DER is not a signature format. ASN.1 is a standard used to describe data structures. DER (Distinguished Encoding Rules) is a specification for ASN.1 structure encoding. Do you any other information on the signature format (raw RSA/PKCS 1, CMS...)?
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