The Mysterious Disappearance of Doctor F. Andrews [Part 1]

Question

This is my first real puzzle, so I hope you enjoy it! Sorry if the writing is a little poor, etc..

You are the unstoppable Detective Joshua Brown, and no puzzle can stand in your way!

The Case:
You awake at once to a phone call from the police: Your best friend since childhood, Doctor Frank Andrews, has been reported missing! Arriving on the crime scene, you search high and low for a clue, when suddenly, you find it.

A handwritten note, taped to the bottom of the mahogany desk in his study. It's not Frank's handwriting. You read it:

"If you wish to see him again, you must prove yourself worthy. The clue to finding Doctor Andrews is hidden behind a painting in this room. Attempting to open the wrong painting will set off an alarm and notify us. We will kill him immediately.".

You shiver in fear: these men clearly mean business. Alongside this note are two more. The first reads:
"u171Bb4C0NBwiOX2e81Luc6S/OWKyDO0QjE5PwiDoF8b4YmxlIA55oGiRm735nNUTKm7ZO+d0Hh3QJb=".

And the second:
"PINVEBLFWFMCCPRWIKWUGUSFZR".

Spurred on by the need to save your friend, you get to work on the case. Frank is an art lover, so there are five replicas of famous paintings on the wall: The Mona Lisa, Basket of Fruit, Sunflowers, The Persistence of Memory and The Scream, but which holds the oh-so-important clue?

Well, that's all. Try to figure out the painting using the three notes, and give a reason with your answer. Good luck!

After this puzzle is solved, I will put up the next part in the series within a few days.

Hint 1:

While the first passage appears to be Base64, it's actually not. With that said, you shouldn't just be guessing blindly as to how to decode it. The answer is hidden there somewhere.

Hint 2:

The first passage should be decoded first. To decode the second, you require the plaintext from the first.

Hint 3:

Apologies for the skewed difficulty, here's one more hint: There is a text string hidden inside the first string. It points towards a common cipher that can then be used to decrypt the first string

The story was shortened after a tip from Travis Kindred (Thanks!)

Answer 1

The clue can be found behind

Basket of Fruit

The first cipher is a Bacon cipher based on the word "bacon" can be found in the ciphertext, and the ciphertext has 80 characters, divisible by 5. The ciphertext can be translated to binary by setting alphabetical characters as "A" and non alphabetical characters as "B".

u171B b4C0N BwiOX 2e81L uc6S/ OWKyD O0QjE 5PwiD oF8b4 YmxlI A55oG iRm73 5nNUT Km7ZO +d0Hh 3QJb=
ABBBA ABABA AAAAA BABBA AABAB AAAAA ABAAA BAAAA AABAB AAAAA ABBAA AAABB BAAAA AABAA BABAA BAAAB
P     L     A     Y     F     A     I     R     F     A     N     D     R     E     W     S

Giving "Play fair F Andrews". "Playfair" probably refers to the playfair cipher.

Sticking FANDREWS as the encryption key to the playfair cipher, PINVEBLFWFMCCPRWIKWUGUSFZR decodes as ISAXCSGREATESTACHIEVEMENTZ.

Ignoring the Z as padding, and considering that the X substitutes for the doubled A (Playfair cipher is not good with doubled letters), the plaintext can be read as "ISAAC'S GREATEST ACHIEVEMENT". This is a reference to the apocryphal story of Isaac Newton discovering gravity when an apple fell on his head. The only artwork that references apples is Basket of Fruit.

---

Old explanation:

Here is the list of artists:

Mona Lisa: Leonardo Da Vinci

Basket of Fruit: Michelangelo Merisi (or Amerighi) da Caravaggio

Sunflowers: Vincent Van Gogh

The Persistence of Memory: Salvador Dali

The Scream: Edvard Munch

The most likely among them is Amerighi da Caravaggio, because the plain text can be written as "IS AC'S GREATEST ACHIEVEMENT" (ignoring X and Z as padding, and the fact that no artist is named Isaac). Then Basket of Fruit holds the clue.

Answer 2

Congrats on making a giant leap! As a puzzler whose first question needed improvement, I want to offer a suggestion. This puzzle looks fun, but it has an issue. It lacks brevity. I suggest shortening the puzzle and making your puzzles stand-alone unless the multi-part format is very important. Let me offer an example:

As a homicide investigator, you've seen some odd things, but this takes the cake. A local art appraiser has been kidnapped, but a clue has been left. The note reads: "u171Bb4C0NBwiOX2e81Luc6S/OWKyDO0QjE5PwiDoF8b4YmxlIA55oGiRm735nNUTKm7ZO+d0Hh3QJb= --- --- --- PINVEBLFWFMCCPRWIKWUGUSFZR".

There are five replicas of famous paintings on the wall: The Mona Lisa, Basket of Fruit, Sunflowers, The Persistence of Memory and The Scream, and your intuition tells you that one of them holds the key to solving this case.

Now, here's where I backpedal. It's possible that the change has broken the puzzle. It's possible that the lengthy narrative is required. I don't know because the puzzle is unsolved. Just think about the value of brevity and let it improve your work in the future. One of the best things you can do in any form of writing is to save your audience time. They'll value you for it.