user: Translate the below assembly code into C programming language:

```asm
<playfair_encipher>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  mov    %rdi,%r13
  mov    %rsi,%rdi
  push   %r12
  mov    %rsi,%r12
  push   %rbp
  push   %rbx
  sub    $0x28,%rsp
  mov    %rdx,0x18(%rsp)
  call   <strlen@plt>
  mov    %eax,0x14(%rsp)
  test   %eax,%eax
  jle    L22
  mov    0x18(%rsp),%rbx
  sub    $0x1,%eax
  mov    %r13d,%r14d
  shr    %eax
  lea    0x2(%rbx,%rax,2),%rax
  mov    %rbx,%r15
  mov    %rax,0x8(%rsp)
  jmp    L112
  nopl   (%rax)
L26:
  cmp    %eax,%ebp
  je     L80
  add    %eax,%edi
  add    %ebp,%esi
  movslq %edi,%rdi
  movslq %esi,%rsi
  movzbl 0x0(%r13,%rdi,1),%eax
  mov    %al,(%r15)
  movzbl 0x0(%r13,%rsi,1),%eax
L11:
  mov    %al,0x1(%r15)
  add    $0x2,%r12
  add    $0x2,%r15
  cmp    0x8(%rsp),%r15
  je     L22
L112:
  movsbl (%r12),%edi
  call   <tolower@plt>
  movsbl 0x1(%r12),%edi
  mov    %eax,%ebp
  call   <tolower@plt>
  movsbl %bpl,%esi
  mov    %r13,%rdi
  mov    %eax,%ebx
  call   <strchr@plt>
  movsbl %bl,%esi
  mov    %r13,%rdi
  mov    %eax,%ebp
  call   <strchr@plt>
  sub    %r14d,%ebp
  movslq %ebp,%r9
  sub    %r14d,%eax
  mov    %ebp,%ecx
  imul   $0x66666667,%r9,%rdx
  sar    $0x1f,%ecx
  movslq %eax,%r10
  mov    %eax,%esi
  sar    $0x1f,%esi
  sar    $0x21,%rdx
  sub    %ecx,%edx
  imul   $0x66666667,%r10,%rcx
  lea    (%rdx,%rdx,4),%edi
  sub    %edi,%ebp
  sar    $0x21,%rcx
  sub    %esi,%ecx
  lea    (%rcx,%rcx,4),%esi
  sub    %esi,%eax
  cmp    %ecx,%edx
  jne    L26
  cmp    $0x4,%ebp
  je     L72
  movzbl 0x1(%r13,%r9,1),%edx
  mov    %dl,(%r15)
  cmp    $0x4,%eax
  je     L120
  movzbl 0x1(%r13,%r10,1),%eax
  jmp    L11
  xchg   %ax,%ax
L80:
  cmp    $0x4,%edx
  je     L15
  movzbl 0x5(%r13,%r9,1),%eax
  mov    %al,(%r15)
  cmp    $0x4,%ecx
  je     L7
  movzbl 0x5(%r13,%r10,1),%eax
  jmp    L11
  xchg   %ax,%ax
L22:
  mov    0x18(%rsp),%rbx
  movslq 0x14(%rsp),%rax
  movb   $0x0,(%rbx,%rax,1)
  add    $0x28,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
  nopl   (%rax)
L72:
  movzbl -0x4(%r13,%r9,1),%eax
  mov    %al,(%r15)
  movzbl 0x1(%r13,%r10,1),%eax
  jmp    L11
  nopl   0x0(%rax)
L15:
  movzbl -0x14(%r13,%r9,1),%eax
  mov    %al,(%r15)
  movzbl 0x5(%r13,%r10,1),%eax
  jmp    L11
  nopl   0x0(%rax)
L120:
  movzbl -0x4(%r13,%r10,1),%eax
  jmp    L11
  nopl   0x0(%rax,%rax,1)
L7:
  movzbl -0x14(%r13,%r10,1),%eax
  jmp    L11
```

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';
}
```