bcrypt merupakan fungsi hashing kata sandi yang dirancang oleh dua orang peneliti keamanan komputer Niels Provos dan David Mazières, cipher Blowfish adalah dasar pembuatan bcrypt, dan disajikan di USENIX pada tahun 1999. bcrypt dapat melindungi dari serangan rainbow table dengan mengunakan salt, selain itu, bcrypt adalah fungsi adaptif: seiring waktu, jumlah iterasi dapat ditingkatkan untuk membuatnya lebih lambat, sehingga tetap aman terhadap serangan pencarian brute-force bahkan dengan meningkatnya daya komputasi.

Fungsi bcrypt merupakan algoritme hash password dasar untuk OpenBSD  dan sistem lain termasuk beberapa distribusi Linux seperti SUSE Linux .

bcrypt dapat diimplementasikan pada bahasa pemrograman PHP, Python, JavaScript, C, C ++, C #, Go, Java, Elixir, Perl, Ruby dan bahasa lain

Algortima 
Algoritme bcrypt adalah hasil dari enkripsi teks "OrpheanBeholderScryDoubt" 64 kali menggunakan Blowfish . Dalam bcrypt fungsi biasa key setup pada Blowfish digantikan dengan fungsi expensive key setup (EksBlowfishSetup)  
 Function bcrypt
    Input:
       cost:     Number (4..31)                      log2(Iterations). e.g. 12 ==> 212 = 4,096 iterations
       salt:     array of Bytes (16 bytes)           random salt
       password: array of Bytes (1..72 bytes)        UTF-8 encoded password
    Output: 
       hash:     array of Bytes (24 bytes)
 
    //Initialize Blowfish state with expensive key setup algorithm
    state  <- EksBlowfishSetup(cost, salt, password)   
 
    //Repeatedly encrypt the text "OrpheanBeholderScryDoubt" 64 times
    ctext  <- "OrpheanBeholderScryDoubt"  //24 bytes ==> three 64-bit blocks
    repeat (64)
       ctext  EncryptECB(state, ctext) //encrypt using standard Blowfish in ECB mode
 
    //24-byte <- ctext is resulting password hash
    return Concatenate(cost, salt, ctext)

Expensive key setup 
Algoritme bcrypt sangat bergantung pada algoritme key setup "Eksblowfish", berikut algoritmanya:
 Function EksBlowfishSetup
    Input:
       cost:     Number (4..31)                      log2(Iterations). e.g. 12 ==> 212 = 4,096 iterations
       salt:     array of Bytes (16 bytes)           random salt
       password: array of Bytes (1..72 bytes)        UTF-8 encoded password
    Output: 
       state:    opaque BlowFish state structure
  
    state  <- InitialState()
    state  <- ExpandKey(state, salt, password)
    repeat (2cost)
       state  <- ExpandKey(state, 0, password)
       state  <- ExpandKey(state, 0, salt)
 
     return state

Expand key 
Berikut algoritme fungsi ExpandKey
 Function ExpandKey(state, salt, password)
    Input:
       state:    Opaque BlowFish state structure     Internally contains P-array and S-box entries
       salt:     array of Bytes (16 bytes)           random salt
       password: array of Bytes (1..72 bytes)        UTF-8 encoded password
    Output: 
       state:    opaque BlowFish state structure
  
    //Mix password into the internal P-array of state
    for n  <- 1 to 18 do
       Pn  <- Pn xor password[32(n-1)..32n-1] //treat the password as cyclic
 
    //Encrypt state using the lower 8 bytes of salt, and store the 8 byte result in P1|P2
    block  <- Encrypt(state, salt[0..63])
    P1  <- block[0..31]  //lower 32-bits
    P2  <- block[32..63] //upper 32-bits
 
    //Continue encrypting state with salt, and storing results in remaining P-array
    for n  <- 2 to 9 do
       block  <- Encrypt(state, block xor salt[64(n-1)..64n-1]) //encrypt using the current key schedule and treat the salt as cyclic
       P2n-1  <- block[0..31] //lower 32-bits
       P2n  <- block[32..63]  //upper 32-bits
 
    //Mix encrypted state into the internal S-boxes of state
    for i  <- 1 to 4 do
       for n  <- 0 to 127 do
          block  <- Encrypt(state, block xor salt[64(n-1)..64n-1]) //as above
          Si[2n]    <- block[0..31]  //lower 32-bits
          Si[2n+1]  <- block[32..63]  //upper 32-bits
     return state

Referensi 

Perangkat lunak kriptografi
Kriptografi
Halaman dengan terjemahan tak tertinjau