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KDF
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Edit File: cdesc-KDF.ri
U:RDoc::NormalModule[iI"KDF:ETI"OpenSSL::KDF;T0o:RDoc::Markup::Document: @parts[o;;[o: RDoc::Markup::Paragraph;[I"FProvides functionality of various KDFs (key derivation function).;To: RDoc::Markup::BlankLine o; ;[ I"LKDF is typically used for securely deriving arbitrary length symmetric ;TI"Nkeys to be used with an OpenSSL::Cipher from passwords. Another use case ;TI"Iis for storing passwords: Due to the ability to tweak the effort of ;TI"Ncomputation by increasing the iteration count, computation can be slowed ;TI"Fdown artificially in order to render possible attacks infeasible.;T@o; ;[I"LCurrently, OpenSSL::KDF provides implementations for the following KDF:;T@o:RDoc::Markup::List: @type: BULLET: @items[o:RDoc::Markup::ListItem: @label0;[o; ;[I"BPKCS #5 PBKDF2 (Password-Based Key Derivation Function 2) in ;TI"combination with HMAC;To;;0;[o; ;[I" scrypt;To;;0;[o; ;[I" HKDF;T@S:RDoc::Markup::Heading: leveli: textI" Examples;TS;;i;I"5Generating a 128 bit key for a Cipher (e.g. AES);To:RDoc::Markup::Verbatim;[ I"pass = "secret" ;TI"-salt = OpenSSL::Random.random_bytes(16) ;TI"iter = 20_000 ;TI"key_len = 16 ;TI"Hkey = OpenSSL::KDF.pbkdf2_hmac(pass, salt: salt, iterations: iter, ;TI"C length: key_len, hash: "sha1") ;T: @format0S;;i;I"Storing Passwords;To;;[I"pass = "secret" ;TI"+# store this with the generated value ;TI"-salt = OpenSSL::Random.random_bytes(16) ;TI"iter = 20_000 ;TI"(hash = OpenSSL::Digest::SHA256.new ;TI" len = hash.digest_length ;TI"$# the final value to be stored ;TI"Jvalue = OpenSSL::KDF.pbkdf2_hmac(pass, salt: salt, iterations: iter, ;TI"? length: len, hash: hash) ;T;0S;;i;I")Important Note on Checking Passwords;To; ;[ I"JWhen comparing passwords provided by the user with previously stored ;TI"Kvalues, a common mistake made is comparing the two values using "==". ;TI"DTypically, "==" short-circuits on evaluation, and is therefore ;TI"Jvulnerable to timing attacks. The proper way is to use a method that ;TI"Jalways takes the same amount of time when comparing two values, thus ;TI"Hnot leaking any information to potential attackers. To compare two ;TI")values, the following could be used:;T@o;;[I" def eql_time_cmp(a, b) ;TI"# unless a.length == b.length ;TI" return false ;TI" end ;TI" cmp = b.bytes ;TI" result = 0 ;TI"& a.bytes.each_with_index {|c,i| ;TI" result |= c ^ cmp[i] ;TI" } ;TI" result == 0 ;TI" end ;T;0o; ;[I"HPlease note that the premature return in case of differing lengths ;TI"Ktypically does not leak valuable information - when using PBKDF2, the ;TI":length of the values to be compared is of fixed size.;T: @fileI"ext/openssl/ossl_kdf.c;T:0@omit_headings_from_table_of_contents_below0;0;0[ [ [ [[I" class;T[[: public[[I" hkdf;TI"ext/openssl/ossl_kdf.c;T[I"pbkdf2_hmac;T@o[I" scrypt;T@o[:protected[ [: private[ [I" instance;T[[;[ [;[ [;[ [ [U:RDoc::Context::Section[i 0o;;[ ;0;0[I"ext/openssl/ossl.c;TI" OpenSSL;TcRDoc::NormalModule
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