user: What is the c source code of the assembly code below:

```asm
<playfair_encipher>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x18,%rsp
  mov    %rdi,%rbp
  mov    %rsi,%r14
  mov    %rdx,%rbx
  mov    %rdx,0x8(%rsp)
  mov    %rsi,%rdi
  call   <strlen@plt>
  mov    %eax,0x4(%rsp)
  test   %eax,%eax
  jle    L90
  mov    %rbx,%r13
  sub    $0x1,%eax
  shr    %eax
  mov    %eax,%eax
  lea    0x2(%rbx,%rax,2),%r15
  jmp    L51
L42:
  cmp    $0x4,%esi
  je     L47
  cmp    $0x4,%edi
  je     L75
  movslq %ebx,%rbx
  movzbl 0x1(%rbp,%rbx,1),%eax
  mov    %al,0x0(%r13)
  movslq %ecx,%rcx
  movzbl 0x1(%rbp,%rcx,1),%eax
  mov    %al,0x1(%r13)
L102:
  add    $0x2,%r14
  add    $0x2,%r13
  cmp    %r15,%r13
  je     L90
L51:
  movsbl (%r14),%edi
  call   <tolower@plt>
  mov    %eax,%ebx
  movsbl 0x1(%r14),%edi
  call   <tolower@plt>
  mov    %eax,%r12d
  movsbl %bl,%esi
  mov    %rbp,%rdi
  call   <strchr@plt>
  mov    %eax,%ebx
  sub    %ebp,%ebx
  movsbl %r12b,%esi
  mov    %rbp,%rdi
  call   <strchr@plt>
  mov    %eax,%ecx
  sub    %ebp,%ecx
  movslq %ebx,%rax
  imul   $0x66666667,%rax,%rax
  sar    $0x21,%rax
  mov    %ebx,%edx
  sar    $0x1f,%edx
  sub    %edx,%eax
  movslq %ecx,%rdx
  imul   $0x66666667,%rdx,%rdx
  sar    $0x21,%rdx
  mov    %ecx,%esi
  sar    $0x1f,%esi
  sub    %esi,%edx
  lea    (%rax,%rax,4),%edi
  mov    %ebx,%esi
  sub    %edi,%esi
  lea    (%rdx,%rdx,4),%r8d
  mov    %ecx,%edi
  sub    %r8d,%edi
  cmp    %edx,%eax
  je     L42
  cmp    %edi,%esi
  je     L113
  lea    (%rax,%rax,4),%eax
  add    %edi,%eax
  cltq
  movzbl 0x0(%rbp,%rax,1),%eax
  mov    %al,0x0(%r13)
  lea    (%rdx,%rdx,4),%eax
  add    %esi,%eax
  cltq
  movzbl 0x0(%rbp,%rax,1),%eax
  mov    %al,0x1(%r13)
  jmp    L102
L47:
  movslq %ebx,%rbx
  movzbl -0x4(%rbp,%rbx,1),%eax
  mov    %al,0x0(%r13)
  movslq %ecx,%rcx
  movzbl 0x1(%rbp,%rcx,1),%eax
  mov    %al,0x1(%r13)
  jmp    L102
L75:
  movslq %ebx,%rbx
  movzbl 0x1(%rbp,%rbx,1),%eax
  mov    %al,0x0(%r13)
  movslq %ecx,%rcx
  movzbl -0x4(%rbp,%rcx,1),%eax
  mov    %al,0x1(%r13)
  jmp    L102
L113:
  cmp    $0x4,%eax
  je     L58
  cmp    $0x4,%edx
  je     L84
  movslq %ebx,%rbx
  movzbl 0x5(%rbp,%rbx,1),%eax
  mov    %al,0x0(%r13)
  movslq %ecx,%rcx
  movzbl 0x5(%rbp,%rcx,1),%eax
  mov    %al,0x1(%r13)
  jmp    L102
L58:
  movslq %ebx,%rbx
  movzbl -0x14(%rbp,%rbx,1),%eax
  mov    %al,0x0(%r13)
  movslq %ecx,%rcx
  movzbl 0x5(%rbp,%rcx,1),%eax
  mov    %al,0x1(%r13)
  jmp    L102
L84:
  movslq %ebx,%rbx
  movzbl 0x5(%rbp,%rbx,1),%eax
  mov    %al,0x0(%r13)
  movslq %ecx,%rcx
  movzbl -0x14(%rbp,%rcx,1),%eax
  mov    %al,0x1(%r13)
  jmp    L102
L90:
  movslq 0x4(%rsp),%rax
  mov    0x8(%rsp),%rcx
  movb   $0x0,(%rcx,%rax,1)
  add    $0x18,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
```

assistant: ```c
void playfair_encipher(char *key, const char *plaintext, char *ciphertext) {
  int i = 0, length = strlen(plaintext);
  char a, b;
  int a_ind, b_ind, a_row, b_row, a_col, b_col;
  for (i = 0; i < length; i += 2) {
    a = tolower(plaintext[i]);
    b = tolower(plaintext[i + 1]);
    a_ind = (int)strchr(key, a) - (int)key;
    b_ind = (int)strchr(key, b) - (int)key;
    a_row = a_ind / 5;
    b_row = b_ind / 5;
    a_col = a_ind % 5;
    b_col = b_ind % 5;
    if (a_row == b_row) {
      if (a_col == 4) {
        ciphertext[i] = key[a_ind - 4];
        ciphertext[i + 1] = key[b_ind + 1];
      } else if (b_col == 4) {
        ciphertext[i] = key[a_ind + 1];
        ciphertext[i + 1] = key[b_ind - 4];
      } else {
        ciphertext[i] = key[a_ind + 1];
        ciphertext[i + 1] = key[b_ind + 1];
      }
    } else if (a_col == b_col) {
      if (a_row == 4) {
        ciphertext[i] = key[a_ind - 20];
        ciphertext[i + 1] = key[b_ind + 5];
      } else if (b_row == 4) {
        ciphertext[i] = key[a_ind + 5];
        ciphertext[i + 1] = key[b_ind - 20];
      } else {
        ciphertext[i] = key[a_ind + 5];
        ciphertext[i + 1] = key[b_ind + 5];
      }
    } else {
      ciphertext[i] = key[5 * a_row + b_col];
      ciphertext[i + 1] = key[5 * b_row + a_col];
    }
  }
  while (i < length)
    ;
  ciphertext[length] = '\0';
}
```