The program asks questions, does the sha1 of the answer and
uses it to decrypt a file using AES (encrypted files are
embedded in the binary by means of xxd). Decrypted files are
memes, and have the next answer written into them by means of
steganography. The flag is in the last file.

This creats a bootstrap problem: how to get the answer to the
first question? Well, you can't. But the the first file is a
bitmap encoded using AES ECB (the other two use CTR). So you
can look through the encryption and see the answer.

Binary was stripped and compressed with upx.

Solution:

$ ./memento
Which question?
0
How to solve the chicken and egg problem?
^C
$ ./memento
Which question?
1
What have I been?
^C
$ ./memento
Which question?
2
What do I need to go ahead?
^C
$ ./memento
Which question?
3
$

Ok, 0, 1 and 2 produce a question.
Reversing the binary and extracting the ECB-encrypted bmp one can get
the first answer:

Tro11d

Using this as the answer to question 1 one gets a nyan cat audio file.
Looking at the spectrum with audacity one gets:

Thek3yt0th3n3xtl3v3l

that is the answer to question 2. This gives you a jpeg with a trollface
comics. Using steghide with an empty passphrase passphrase you finally get:

$ steghide info 2.jpg
"2.jpg":
  format: jpeg
  capacity: 6.1 KB
Try to get information about embedded data ? (y/n) y
Enter passphrase:
  embedded file "flag.txt":
    size: 62.0 Byte
    encrypted: rijndael-128, cbc
    compressed: yes
$ steghide extract -sf 2.jpg
Enter passphrase:
wrote extracted data to "flag.txt".
$ cat flag.txt
MGZmMzE1MTI5YjkxNGYzZjk1NzFjNDc0ODIxYzI0ZmUxYzE3YTEzMiAgLQo=

Key is in base64, last step is:

$ cat flag.txt | base64 -d
0ff315129b914f3f9571c474821c24fe1c17a132

Hints:
The file obtained through question N contains the answer of question N+1